In recent years, from the viewpoint of dealing with environmental issues such as reduction of carbon dioxide emission, there has arisen an urgent need that automobiles, construction machines and agricultural machines consume less energy, i.e., are reduced in the fuel-consumption thereof. In particular, there is a growing demand that their units such as engines, transmissions, final reduction gear units, compressors and hydraulic equipment contribute to energy saving. Therefore, lubricating oils used in these units have been demanded to be less in frictional loss by agitation and frictional resistance than ever before.
Lowering the viscosity of a lubricating oil may be exemplified as a means for allowing a transmission and a final reduction gear unit to contribute to fuel saving. For example, an automobile automatic transmission or continuously variable transmission has a torque converter, a wet clutch, a gear bearing mechanism, an oil pump and a hydraulic control system while a manual transmission or final reduction gear unit has a gear bearing mechanism. Lowering the viscosity of a lubricating oil to be used in such transmissions can reduce the frictional loss by agitation and frictional resistances of the torque converter, wet clutch, gear bearing mechanism and oil pump and thus enhance the power transmission efficiency thereof, resulting in an improvement in the fuel economy performance of the automobile.
However, when lubricating oil to be used in these transmissions is lowered in viscosity, it will be excellent in low-temperature viscosity characteristics but will be extremely increased in evaporation loss and poor in lubricating properties. As the result, seizure occurs in the engine or transmission and thus may cause some malfunctions therein.
Examples of conventional automobile transmission oils which enables a transmission to maintain various properties such as shifting properties for a long time include those produced by, optimizing and blending synthetic and/or mineral base oils, antiwear agents, extreme pressure additives, metallic detergents, ashless dispersants, friction modifiers and viscosity index improvers (for example, see Patent Document Nos. 1 to 4 below). However, these compositions are not aimed at improving the fuel saving performance of an automobile and thus are high in kinematic viscosity. Any of the documents does not refer to effects on lubricating properties obtained by lowering the viscosity of lubricating oil at all. Therefore, a composition which can solve the foregoing problems has not been sufficiently studied yet.    (1) Japanese Patent Application Laid-Open Publication No. 3-39399    (2) Japanese Patent Application Laid-Open Publication No. 7-268375    (3) Japanese Patent Application Laid-Open Publication No. 2000-63869    (4) Japanese Patent Application Laid-Open Publication No. 2001-262176